Because humans have an inborn ability to think of themselves. I mean think of themselves as individuals, solve problems in ways other animals don't, and grasp concepts they don't understand. It's a term that we apply to ourselves because so far humans are the only proven creatures to even have a term for it. I respect your opinion, even if I don't think you're right.

"Sentience is the ability to feel, perceive, or to experience subjectivity."
"Subjectivity is a term used to refer to the condition of being a subject: i.e., the quality of a subject's perspective, experiences, feelings, beliefs, and desires."

The knowledge gained from perspective, experiences, feelings, beliefs and desires is huge and varying, from how to disembowel your dinner to how to fix your house. A sentient being would be able to shift and place priorities on various experiences. A student learning would try to remember what they learnt, for example.

Technically speaking along this line of thought, monkeys are sentient. In fact, there was that one monkey that lied to her handlers in sign language about a kitten being to blame for tearing a sink out of a wall.

Alternatively, it was the advent of Perceived Value, i.e. money, that changed how things ran. Farmers don't need to worry about inner-city gangsters coming to murder them.
Value and transfer of it has altered humanity's ecosystem greatly, now that I think about it.

So say there's a chicken. Just a regular chicken, possibly a wild chicken if those are still around. It is killed by a predator and eaten. If a human is killed by a predator or another human, the response is different from the response if a chicken is killed by a predator. So by default they aren't the same. I don't think society would have advanced this far if people were too opposed to eating meat. That's why we're omnivores.

But it also produces positive feedback in the people who profit from it, and the people who live off of it, doesn't it? Meat can often be cheaper than plants when it comes to buying, and people who otherwise cannot get food get their food from the meat industry. That would make it a positive system as well, wouldn't it?

I think you're overlooking the fact that people can up and die from not getting their food from meat, especially because it's cheaper. And yes, meat is pretty delicious. Steak is especially delicious rare. As in, I would murder a firstborn child to get myself some steak if I was super poor.

So we can summarise that:
Any living organism, if sufficiently complex in its actions, is sentient.
The 'Sentience threshold' is passed when an organism or species is able to learn and place value upon subjectivity.

... And to my knowledge, it's just those two points. A question, though: What did you mean about 'negative' impact? All I've seen so far is impact which affects X, Y and Z, through means A, B and C. Is the entire environment easily classified as a 'positive' or 'negative' one?
Technically speaking, Pripyat (for the sake of examples) should be the most negative environment ever. But life still thrives there. Just not human life.

When I say sentience I'm referring to the awareness you have, like a computer could mimic the ability to apply subjective value without being sentient. You can be objective and sentient in the way I define it, as long as you experience that objectivity within your field of awareness.

I believe that an environment is negative when it contains negative components (suffering) in a greater magnitude than it contains positive components. It doesnt matter how dirty or subjectively awful it is, it's value is defined objectively by its components.

Ah. I see. I was going on the line of being able to independently and/or spontaneously assign Perceived value to something. Like any person would assign value to jewelery because we would perceive it as valuable.

Again, what's your definition of 'negative' and 'positive'? Is it strictly centered around all people within the environment? In this case, the United States could be considered a highly negative environment that is regardless rich in resources.

And is your mention of 'suffering' subjective? Whose 'suffering' is it, in other words?

So you take a system containing two people. Person A experiences value of magnitude "-1" and person B experiences "+1" . We don't have these values in real life obviously because we can't compare the magnitudes but we know the magnitudes exist (self-evident) . By that situation I mean they are both experiencing states of opposite value so over all that system is objectively neutral, even though persons A and B may argue about their subjective view of the system, they are both objectively in opposite states and the system is objectively neutral.

All in all, the system isn't too heavily affected. Even if it is, it'll either find some way to recover or change to keep functioning or it'll break apart and be swallowed up by other systems.
A+1 and B-1 would still mean that system AB has a value of 0.

I think we were on the same page to begin with. What we have experienced, tovarisch, is a failure to communicate.

Suffering is a process that sentient things go through and it's self-evidently negative in value. (It's negative value can be assessed just by experiencing it in isolation) /
The opposite is true for pleasure.
This isn't subjective, they have definite magnitudes that can be compared to eachother and if given a reference point you could measure a systems value it just like length or weight. We just don't have any tools to compare them.